Strangles

How to spot signs of this highly contagious bacterial respiratory infection, then take steps to treat it and contain its spread.

What It Is

Strangles is a highly contagious bacterial infection of the upper respiratory tract in horses. It is characterized by swelling of the lymph nodes under the jaw or in the throat area and the formation of abscesses that may become large enough to obstruct the airway and affect a horse’s breathing (thus the term “strangles”).

A strangles-like disease was described long ago in Roman circus horses and later, in the mid-1200s, in Germany. It occurs worldwide, and in 2017 became a reportable disease in the United States because of its contagiousness and frequency. Horses of all ages—as well as donkeys and mules—may develop strangles. Youngsters from 1 to 5 years old generally experience the most severe infections. Foals younger than 4 months gain some protection through antibodies they receive in their dam’s colostrum.

Most horses recover from strangles with only supportive care and develop strong protective immunity that lasts for about a year, often much longer. The condition can take three to six weeks to resolve. Some horses who appear healthy may continue to harbor and intermittently shed the bacteria.

Strangles is rarely fatal in otherwise healthy horses. However, life-threatening complications can occur. Metastatic strangles (“bastard strangles”) is associated with abscesses in lymph nodes in the abdomen and chest. A rare complication is purpura hemorrhagica, with immune-mediated damage—caused by the immune system mistakenly targeting the body—to blood vessels throughout the body.

Cause

Strangles is caused by Streptococcus equi, a bacterium that spreads from horse to horse through direct contact. It is also transmitted via contaminated equipment: water buckets, tack, blankets, grooming tools, stall-cleaning implements, trailers—virtually any item in an infected horse’s vicinity. This includes bedding and feed. Caretakers, too, may pass the bacteria, which can cling to hands and clothing. Streptococcus equi can stay active in moist conditions for four to six weeks and survives only briefly in drier areas, for instance on fences or in soil, for one to three days.

Signs

The incubation period for strangles is approximately three to 14 days after exposure. The first noticeable sign is sudden fever as high as 106 degrees Fahrenheit. Clear discharge likely will begin draining from the horse’s nose. It will turn thick and white to yellow in color. A few days after the onset, the lymph nodes under the jaw will swell and become painful. Eventually, these abscesses will rupture, draining through the skin or into the guttural pouches (large air-filled sacs in the throatlatch region). A horse with strangles may hold his head and neck in an abnormal position because of the abscesses. Other signs may include:

difficulty swallowing
labored breathing
coughing
loss of appetite
lethargy
depression.

*A common sign of strangles is swollen lymph nodes under the jaw, which will rupture and drain through the skin. ©Amy K. Dragoo*

Diagnosis

If you suspect that your horse has strangles, call your veterinarian as soon as possible. A bacterial culture of secretions from an abscess or a nasal swab or wash will confirm whether a horse has strangles. Culture is less sensitive than commonly used PCR tests, which detect DNA of S. equi. However, a positive PCR test is not proof of the presence of infectious bacteria.

Management

Start by isolating the sick horse. Identify any others who may be infected by checking their rectal temperature, and then isolate them, too, if they have a fever. A separate barn is ideal. If that’s not available, use a stall that is 15 to 20 feet away rom other horses. Designate specific items for a sick horse’s use; don’t share them with others. Use household bleach or a cleaning product containing phenol to disinfect equipment that might have been contaminated. Handle a sick horse last if you are caring for others. Wear disposable gloves when tending to him and wash your hands afterward. Change clothes, including shoes, before coming in contact with other horses. Continue to be cautious until a test sample returns clean.

Treatment

Your veterinarian will recommend a treatment strategy based on the severity of your horse’s infection. For an uncomplicated case of strangles, she’s likely to prescribe supportive care:

Shelter the horse in a comfortable, dry, well-ventilated environment that’s neither too hot nor too cold.
Provide palatable, soft feed that’s easy to swallow.
Apply warm compresses to swollen areas to encourage the abscesses to drain.
Flush ruptured abscesses with dilute povidone iodine solution (3 to 5 percent) for several days until drainage ceases.

In some cases, a veterinarian may prescribe a nonsteroidal anti-inflammatory medication to decrease pain and fever and increase appetite. But unless your horse’s infection is severe, she’ll likely refrain from recommending antibiotics because they can prolong recovery and interfere with the infected individual’s ability to develop a strong immune response to prevent reinfection.

Prevention

Medical: Two types of strangles vaccine are licensed for use in the U.S.: an intramuscular injectable and an intranasal form. Neither eliminates the risk of developing the disease, but either one can lessen the severity of an infection. Your vet can discuss the pros and cons with regard to your horse’s circumstances and potential risk of exposure. An initial dose of either type is administered to a healthy horse and followed by a booster three to four weeks later.

Management: Good caretaking practices at a stable or farm can significantly reduce the chances of a horse developing strangles and then infecting others on the premises. It’s advisable to:

have sufficient space indoors and outside to avoid overcrowding
take steps to ensure that housing is clean and dry
be conscientious about the cleanliness of equipment, especially if it’s shared, such as feeders and water troughs
manage manure responsibly
control rodents to avoid disease spread
quarantine any horse new to the property for three weeks.

There are additional precautions you can take if your horse routinely travels to shows and events:

Avoid mingling with other horses, particularly those you don’t know.
Minimize the use of shared stalls and pastures.
Bring feed, buckets and equipment from home and designate them specifically for your horse’s use.
Consider that travel may stress your horse and decrease his body’s ability to fight illness.
Stay up to date on the location and extent of recent reported outbreaks.

Practical Horseman thanks John F. Timoney, MVB, PhD, for his technical assistance in the preparation of this article. Dr. Timoney is professor emeritus at the University of Kentucky Gluck Equine Research Center. His specialties include equine bacterial diseases, immunology and equine education and research.

This article originally appeared in the Winter 2024 issue of Practical Horseman.